1. Field of the Invention
The invention relates to antenna systems for receiving and/or transmitting electromagnetic signals. More specifically, the invention relates to loop antenna systems with peripheral gaps.
2. Description of the Prior Art
In the prior art the loop antenna per se, has been a familiar device. Such antennas have been used for direction finding and for certain types of frequency surveillance, since they are inherently (at least as usually applied) non-resonant and relatively broadband in their operation.
The familiar vertically oriented (i.e., oriented with the plane of the loop in a vertical plane) is particularly well known in a number of variations, and has long been used in a form rotatable in the azimuth plane for null-seeking, direction-finding applications. In such cases, the radiation/receiving direction is in a plane normal to the plane of the loop, or, stated otherwise, normal to the vertical axis about which the loop is rotated in the azimuth coordinate.
A number of variations in the loop antenna art are known, such as the so-caled automatic direction finding (ADF) antennas used for aircraft direction finding and homing which utilize a multi-turn loop in conjunction with a separate monopole sense antenna in a system calculated to provide non-ambiguous bearing determination with respect to predetermined fixed radio wave sources.
Loop antennas have also been applied in antenna systems intended for transmission/reception along directional vectors lying in the plane of the loop. Accordingly, the plane of a loop intended for that type of operation would be horizontal.
In certain aerospace antenna systems, particularly in the VHF/UHF frequency regions, the pattern of an antenna when mounted in close proximity to the vehicle structure is severely distorted from its free space pattern due to the coupling to, and reflections from, the aerospace vehicle and its apendages. The achievement of the advantages of a loop antenna in respect to broadband operation, lightness and small size, are in such cases unfavorably counterbalanced by such interaction and reflections.
Still further, loop antenna systems employing separate dipoles or the like for ambiguity resolution in direction finding, suffer from the effects of separation of phase centers.
Certain other known devices such as multimode log-spiral antennas and multiple horn arrays can provide monopulse direction finding and guidance capability, however, those devices suffer a number of deficiencies when there is a firm requirement for minimum size and weight in an antenna system. Frequently an antenna system must be small relative to a wavelength at the lowest frequency of operation (&lt;0.1 .lambda.).
In consideration of the foregoing general problems of the state of the art, there has been a clear need for an antenna system concept which permits retention of the particular advantages of the "edge-on" receiving loop antenna, without the disadvantages previously encountered in connection with such devices. The manner in which the present invention deals with the prior art situation to produce a particularly novel and useful loop-type antenna system will be understood as this description proceeds.
Loop antennas employing peripheral gaps have been known in the prior art for the purpose of making RF current distribution more uniform about the loop perimeter, however such prior art devices have not included the structure of the combination of the invention.